Add normal_strain boundary condition type (for elasticity)

It is treated as a Dirichlet BC type, not sure if it is the
appropriate one ...

src/language/modules/SchemeModule.cpp

@@ -96,6 +96,19 @@ SchemeModule::SchemeModule()
 
                               ));
 
+  this->_addBuiltinFunction("normal_strain",
+                            std::make_shared<BuiltinFunctionEmbedder<std::shared_ptr<
+                              const IBoundaryConditionDescriptor>(std::shared_ptr<const IBoundaryDescriptor>,
+                                                                  const FunctionSymbolId&)>>(
+
+                              [](std::shared_ptr<const IBoundaryDescriptor> boundary,
+                                 const FunctionSymbolId& g_id) -> std::shared_ptr<const IBoundaryConditionDescriptor> {
+                                return std::make_shared<DirichletBoundaryConditionDescriptor>("normal_strain", boundary,
+                                                                                              g_id);
+                              }
+
+                              ));
+
   this->_addBuiltinFunction("fourier",
                             std::make_shared<BuiltinFunctionEmbedder<std::shared_ptr<
                               const IBoundaryConditionDescriptor>(std::shared_ptr<const IBoundaryDescriptor>,